1. Field of the Invention
The invention relates to a cutting tool, such as a twist drill, having a uniquely oriented chisel edge.
2. Description of Related Art
It is important in the drilling of brittle materials such as grey cast iron to drill a circular hole at a desired location and in a straight direction with “hole straightness”. During the drilling process, not only is it possible for the drill to “walk” across the surface of a workpiece away from the target location but, furthermore, even when the drill is positioned at the proper target location, asymmetrical cutting forces upon the drill may cause the drill to wobble, thereby producing a non-circular hole, a hole that is not straight, or both. U.S. Pat. No. 3,977,807 issued Aug. 31, 1976, is directed to a double margin twist drill having a chisel edge that forms a chisel edge angle with the corner of the primary margin. While FIG. 2 of this patent illustrates this feature, it is difficult to discern the chisel edge angle because FIG. 2 is a perspective view. Nevertheless, the outer diameter of the primary margin reduces as the margin approaches the chisel edge and, as a result, some drill stability is sacrificed.
U.S. Pat. No. 7,267,514 issued Sep. 11, 2007, is directed to a self-centering bit drill with a pilot tip, wherein the drill has only a single margin. This patent discloses in FIG. 4, an arrangement, whereby the chisel edge is essentially parallel with the drill margin. While this reduces the “walking” of the drill, the same problem exists with respect to hole shape and hole straightness because the drill tends to wobble while drilling.
A drill design is needed that makes straighter and better located holes, and that stabilizes the drill within the hole during the hole making process.